The invention relates to a novel method for making a viewing screen structure for a cathode-ray tube and more particularly to a method for depositing a barrier coating and a plurality of coatings layers of a heat absorptive material on a metal layer overlying the viewing screen.
One type of cathode-ray tube that is used for television displays is referred to as a shadow-mask tube. This tube is comprised of an evacuated envelope having a viewing window, a viewing-screen structure comprised of a mosaic of phosphor areas (usually dots or strips) of different emission colors supported on the inner surface of the viewing window, a shadow mask having an array of apertures therein in register with the phosphor areas mounted in the tube in adjacent spaced relation with the window, and means for projecting one or more (usually three) electron beams towards the screen for selectively exciting the phosphor areas thereof.
In operating a shadow-mask tube, the electron beams are made to scan a raster in a fixed pattern. As the beams are made to scan, they are either intercepted by the mask or they pass through the mask apertures and excite the desired phosphor areas. The energy in the intercepted electron beams heats the mask and causes the mask to become distorted, which may adversely affect the position of the beams which pass through the mask apertures. Some of the heat in the mask is removed by radiation back to a dark coating on the funnel of the tube. Normally, the viewing-screen structure includes a thin layer of a highly reflective metal, usually aluminum, which reflects heat that is radiated forward towards the screen.
U.S. Pat. No. 3,703,401 issued to S. B. Deal et al. on Nov. 21, 1972 and U.S. Pat. No. 4,025,661 issued to J. J. Moscony et al. on May 24, 1977 suggest applying to the reflective metal layer on the viewing screen a water based heat-absorptive overcoating of carbon particles. U.S. Pat. No. 4,623,820 issued to S. B. Deal et al. on Nov. 18, 1986 suggests that silica particles be added to the carbon particle overcoating as a binder therefor. The viewing screen structure is baked to remove organic and volatile materials therefrom. The purpose of a heat-absorptive overcoating is to promote the transport of heat from the shadow mask to the atmosphere through the glass panel and thereby reduce mask warpage due to uneven heating of the mask-frame assembly of the tube. Common formulations used in applying these overcoatings include such constituents as finely-divided particles of graphite, lamp black and silica together with dispersants and wetting agents.
The Deal et al. and Moscony et al. patents suggest air spraying the overcoating as well as an initial sealer coating or barrier layer which prevents carbon in the overcoating from penetrating through the aluminum metal layer into the phosphor mosaic. The patents also suggest the use of a suitable shield to prevent the barrier layer and the overcoating from being sprayed on the inner sidewall of the panel and onto the seal land. It is necessary to prevent the coatings from contacting the seal land since contaminants on the seal land will adversely affect the quality of the subsequent frit seal which is required to attach the faceplate panel to the funnel portion of the tube.
As described in copending U.S. patent application Ser. No. 936,500 filed on Dec. 1, 1986 by S. B. Deal and assigned to the assignee of the present invention, the panel may be positioned horizontally during the spray step with the guns positioned below the panel. Alternatively the spray guns may be located above the horizontally disposed panel or the guns may be located at one side of a vertically disposed panel.
In U.S. Pat. Nos. 3,703,401, 4,025,661 and 4,623,820 only one gun is used to spray the barrier coating and about 10 passes of the spray across the surface are required to obtain a satisfactory barrier coating. The sprayed barrier coating typically requires 1 to 3 minutes of air spraying with an air-spray gun operating at about 50-pounds-per-square-inch pressure. The sprayed barrier coating dries in less than a minute, due in part to the heat in the preheated panel. Then, with the panel still preheated above 50.degree. C., the carbon-containing overcoating is sprayed on the previously barrier-coated metal layer. The spraying is conducted for about 2 to 5 minutes with an air-spray gun operating at about 50 pounds-per-square-inch pressure and includes about 20 passes to provide a coating weight of about 0.15 mg/cm.sup.2. The sprayed material dries in less than a minute due in part to the heat in the preheated panel, and forms a heat absorptive overcoating.
A problem with the above-described spraying processes is that the time required to spray the barrier coating and the overcoating is unacceptably long. Additionally, greater coating uniformity is required than has previously been obtained.